It is known in the art to use precast concrete building systems in the construction of culverts and bridges. The structures built according to these systems are composed of one or more elemental sections successively placed adjacent to one another. In this regard the individual sections are placed side-by-side in the ground to form, for example, a bridge beneath traffic-ways for road-over-road or road-over-stream crossings. The elemental sections can also be used to construct culverts and underground storage vaults. By precasting multiple sections offsite for subsequent erection onsite, overall project construction time can be compressed as compared to cast-in-place concrete structures. Also, due to the improved quality control associated with facility manufacturing, the end-product structure provides greater inherent durability over that of a cast-in-place concrete structure. Bridges, culverts and underground storage structures can be made from the same elemental sections. Accordingly, in this application the word “bridge” or “bridge assembly” is defined to include a culvert or underground storage structure, unless otherwise indicated.
There are two basic varieties of prior art precast bridge systems: flattop and arched-top. FIGS. 1a and 1b respectively depict the elemental sections of these prior art systems. As seen in FIGS. 1a and 1b, both the flattop and arched-top section comprise a structural top member 2 integrally connected to and abridging two spaced-apart legs 4. The arched-top bridge section of FIG. 1b is considered by many to be esthetically more pleasing than the flattop section of FIG. 1a. On the other hand, the existing flattop system beneficially maximizes waterway area for water flow of streams and creeks. Structurally, however, the arched-top bridge system is generally more efficient than the flattop systems. The arched-topped section has the ability to carry vertical loads through arching. This arching creates significant outward horizontal thrust in the legs that results in a structure highly dependent on the support of adjacent backfill. By comparison, the prior art flattop bridge sections have higher bending moments and no arching. The flattop section is less dependent on the support of adjacent backfill for structural integrity. However, both the prior art flattop and arched-top bridge systems derive a significant degree of their structural capacity from the support provided by backfill material and are thus susceptible to foundation movements and shifting because of poorly placed backfill. Also, in the case of the arched-top system, in order to manufacture a wide range of spans and rises, a manufacturer must have several different sets of concrete forms on hand. These forms are expensive to manufacture and difficult to use in a production driven environment.